Fred Fehsenfeld looks out a window aboard Miss Piggy, a federal hurricane-research plane, and nods at the gray-brown haze staining the popcorn clouds rising in the midday heat.

"You know, people move to Nashville thinking the air will be clean," the courtly atmospheric chemist drawls. "But it's anything but clean."

Flying for hours at altitudes low enough to count cows at the watering hole, Miss Piggy will log 1,500 miles on this run. The researchers hunched over consoles down her cabin are part of a small army of scientists who converged on Nashville, Tenn., this summer to solve the puzzle of smog in the Southern United States and how best to battle it.

It has become increasingly clear, researchers say, that air-pollution control strategies based on conditions in Los Angeles or the Boston-to-Washington corridor may not help burgeoning Sun Belt cities such as Atlanta or Nashville meet tighter federal air-quality standards.

Illustration: Ari Denison

The Environmental Protection Agency adopted more-stringent ozone rules and tighter regulations for dust particles, which can come from natural and man-made sources. Both standards, adopted two years ago, are tied up in federal court.

"Air-quality management in this country is a remarkable success," notes Dr. Fehsenfeld, of the National Oceanic and Atmospheric Administration's Aeronomy Laboratory in Boulder, Colo., who is an associate director at the University of Colorado's Cooperative Institute for Research in Environmental Sciences. "Cities would almost be unbearable without it. But air quality management is not one size fits all."

"We've been struggling in the US since 1990 to manage ozone and air quality," adds Ellis Cowling, a forest biologist at North Carolina State University in Raleigh, and director of the Southern Oxidants Study (SOS), the program conducting the Nashville experiment. "We've made some progress. The peaks in ozone concentrations are down. But average ozone concentrations have been growing at the same time." This is despite billions of dollars spent on air-pollution control efforts during the past decade.

Since 1988, the SOS project - a cooperative program involving some university scientists, state and federal agencies, and private companies - has been conducting field experiments to get a better handle on the factors contributing to the South's smog problem. The Nashville field campaign represents the most ambitious effort the project has yet undertaken.

The first Nashville campaigns were conducted during the summers of 1994 and 1995. Results from those studies yielded such surprises that the project targeted the city for a six-week effort this year.

Two broad factors separate the Southern US from much of the rest of the country in the battle against urban smog, Dr. Cowling says: weather and trees.

Summer-like weather lingers far longer in the South, where scorching daytime temperatures and stagnant air turn the first several thousand feet of atmosphere here into a solar-powered chemical factory. In urban areas, sunlight provides the energy for reactions that combine oxygen molecules with carbon monoxide, or with hydrocarbons from sources such as auto exhaust, to form ozone.

But since the late 1980s, researchers have grown increasingly attuned to the role tree-made hydrocarbons play in forming ozone over rural areas. Either the hydrocarbons or the ozone itself can carry to cities, compounding urban ozone woes.

Trees as culprit

The key forest-based hydrocarbon in the South is isoprene, which is produced in copious amounts by oaks and members of the poplar family, according to Russell Monson, a professor of environmental biology at the University of Colorado. Research in the region has shown that even forests dominated by pine, which emits its own brand of hydrocarbon, produce vast amounts of isoprene. It comes from plants that make up the pine forest's ground cover.

"Why plants produce isoprene remains a bit of a mystery," Dr. Monson says. One notion holds that the compound stabilizes membranes in plants struggling to sustain photosynthesis during hot weather. This explanation, he adds, is controversial; recent experiments have cast doubt on it.

Photo: AP / LA Times

It's Midday. Can you see City Hall?Los Angeles has cleaned up its act considerably since this photo was taken in 1953.

Isoprene's contribution to ozone production in the presence of a stew of other man-made gases became apparent after strict controls on auto hydrocarbon emissions did little to reduce urban ozone in the South.

"The problem is that forests around many Southern cities have so many oaks, and isoprene is many times more reactive with the atmosphere than man-made hydrocarbons," Monson explains. "This meant that although monitoring programs showed a significant decline in hydrocarbon emissions from autos, ozone didn't go away."

Faced with that conundrum, researchers took to the field. And nowhere, researchers say, do air-pollution conditions unique to the South appear as tidily as they do here at the home of the Grand Ole Opry - a growing urban island surrounded by a sea of woods, whose region includes several coal-fired power plants owned by the Tennessee Valley Authority, one of the participants in the Nashville studies.

Headquartered on the second floor of a nondescript two-story building at Nashville International Airport, the research effort includes ground stations in downtown Nashville and throughout central Tennessee. One site, on a farm within the city limits, has the look of a high-tech Woodstock camp.

As Miss Piggy bumps and sways her way through the unstable air along today's research track, Fehsenfeld reflects on key 1994-1995 results that brought him and his colleagues back to Nashville. The first involves nitrogen oxides, byproducts of combustion without which ozone would not form, he says. Large "point sources" for nitrogen oxides, such as big power plants, turn out to be less efficient at producing ozone than smaller sources, because the rate of ozone production is sensitive to concentrations of nitrogen oxides. He likens it to burning fuel in a car's engine; the fuel burns most efficiently when air and fuel mix in the right proportions.

From a policy standpoint, he continues, the results imply that targeting big power plants for nitrogen-oxide reductions may provide "less bang for the buck" than going after smaller point sources or dispersed sources such as automobiles. Likewise, dealing with emissions of nitrogen oxides by building lots of small power plants instead of one big one could aggravate ozone problems, he notes.

In another key result, he says, scientists were surprised to find that on a regional basis, "isoprene dominates photochemical reactions" forming ozone. Isoprene continues to baffle the research team this year.

"For the first time, we've seen very high concentrations of isoprene at night, and we don't understand it," says Nashville study director James Meagher, also from NOAA's Aeronomy Laboratory. "We know that plants do not produce isoprene at night."

Perhaps it blows in from somewhere else, such as the Great Smoky Mountains, he speculates. Regardless of its source, nighttime concentrations provide a ready reservoir of hydrocarbons to begin the next day's ozone production.

This year marks the first time the project has included particulate studies. The effort is a response to the new EPA regulations, which cite health and aesthetic reasons for wanting to reduce dust haze, as well as to uncertainties about the role particulates play in atmospheric chemistry.

Although researchers often see tiny particles as necessary "platforms" on which chemical reactions take place in the atmosphere, "particulates are a big unknown," says University of Denver atmospheric chemist Charles Brock. "They are something to throw in as answers to holes in our understanding of chemical reactions. But it's equally likely that the holes result from measurement errors."

Dust from Africa

Regardless of the role particulates play, controlling them could be challenging. A 23-year study of dust over Miami indicates that during some summers, up to 50 percent of the fine particles present comes from Africa, not from local sources, according to Joseph Prospero, an atmospheric chemist at the University of Miami. The dust is transported by the same circulation patterns that steer hurricanes across the Atlantic.

His study, published in the July 20 issue of the Journal of Geophysical Research, plus evidence from satellites and more-limited studies elsewhere in the Southeast, led him to conclude that the problem is not limited to Miami, but could well embrace much of the Southeast.

Moreover, those concentrations can reach levels that "could put a city over the top" for dust regulations, even though dust from local sources alone wouldn't constitute a violation.

"Air-quality people need to be aware that African dust is a pretty substantial component" of the Southeast's summertime dust problem, Dr. Prospero says.

Indeed, researchers say, the message that SOS studies, as well as others, continue to drive home is that pollution, or the precursor chemicals that lead to it, know no boundaries.